The present invention relates generally to humidification of a fuel cell, and more particularly to recirculating fluid in a fuel cell anode flowpath to control the level of humidification within a cascaded fuel cell stack.
In a typical fuel cell system, hydrogen or a hydrogen-rich gas is supplied through a flowpath to the anode side of a fuel cell while oxygen (such as in the form of atmospheric oxygen) is supplied through a separate flowpath to the cathode side of the fuel cell. In one form of fuel cell, called the proton exchange membrane (PEM) fuel cell, an electrolyte in the form of a membrane is sandwiched between the anode and cathode to produce a layered structure commonly referred to as a membrane electrode assembly (MEA). Each MEA forms a single fuel cell, and many such single cells can be combined to form a fuel cell stack, increasing the power output thereof. The humidity level in and around the MEA must be controlled to ensure proper fuel cell operation. Water produced during the electrochemical reaction of hydrogen and oxygen, as well as humidification of these reactants prior to their introduction into the fuel cell, can be used to effect such humidity control.
In a conventional single-pass fuel cell stack arrangement, fuel is distributed through a common manifold in a substantially concurrent fashion to each fuel cell in the stack such that each hydrogen-bearing flowpath receives approximately the same concentration of fuel. One disadvantage of such a configuration is that it is difficult to realize thorough reactant utilization. To improve the fuel efficiency, the stack can be arranged as a cascade (also referred to as a multi-pass fuel cell), where the individual cells are divided up into multiple groups (or stages) such that the reactants are supplied concurrently within each group and sequentially between groups. Cascaded fuel cells have the advantage of requiring a lower overall stoichiometry in the anode portion of a fuel cell stack, as smaller quantities of fuel are required to achieve the same useful power output, thereby promoting more efficient operation. As with traditional fuel cell configurations, it is desirable to minimize the number of groups in cascaded fuel cells to simplify and reduce the amount of ancillary equipment (including sensors and flow control componentry). In addition, having fewer groups reduces the likelihood of fuel concentration imbalance in the latter groups, where fuel concentration tends to decrease under the reactions of each successive stage.
One difficulty associated with cascaded fuel cells with a small number of stages (for example, two) is that prohibitively high anode flow is required to ensure adequate hydration levels in the membrane and cathode. One method of meliorating some of these hydration deficiencies is by external humidification, including condensers, water injection and separate water reservoirs. Shortcomings of external humidification approaches include freeze complications in cold environments, as well as greater system complexity associated with the additional componentry. Another approach involves placing more than the stoichiometric amount of fuel in the anode flowpath. While this is helpful in increasing water levels in the membrane, it has the disadvantage of dumping excess fuel overboard, thereby lowering the very utilization that cascaded systems were created to improve.
Still another potential method of ensuring adequate levels of hydration includes humidifying one or both of the reactants before they enter the fuel cell with a water vapor transfer device. In such a device, the water produced at the moisture-rich later stages of the cathode can be extracted and reintroduced (typically in vapor form) into portions of the anode flowpath, cathode flowpath or both where there is little or no moisture. Fibrous tubes, water-permeable membranes or similar devices capable of providing capillary or related water transfer action can be used to effect the transfer of moisture from one stream to the other, but can significantly add to the cost of the system. In addition, measuring and controlling water vapor transfer device humidity output has proven to be difficult.
Efficient operation of a fuel cell system employing one of the approaches discussed above is further hampered when the system requires a source of power to operate. These and other disadvantages are especially troublesome for vehicle-based fuel cell applications, as the often redundant componentry would take up precious vehicle space otherwise used for passenger, comfort or safety features, while the reductions in overall system efficiency impact cost savings. Accordingly, there exists a need for an integrated approach to hydrating a cascaded fuel cell to avoid the cost, complexity or operability associated with other supplemental or traditional humidification approaches.